It has been known to probe materials, such as reinforced concrete, by electrical impedance measurement devices. Such devices comprise a coil and a pulse generator for feeding current pulses to the coil. After each current pulse, the magnetic field generated by the coil decays and gives rise to a decaying induction voltage over the coil. The decay of this voltage is a function of the coil's impedance, which depends on the permeability μ and conductivity σ of the material within the field's reach. For example, if the coil is close to a metallic reinforcement bar embedded within the concrete, the average permeability μ and conductivity σ within the field's reach and therefore the impedance of the coil is changed, which leads to a slower decay of the induction voltage.
Hence, by measuring a parameter indicative of the inductance of the coil, it is possible to gain insight into the composition of the material adjacent to the coil. This is particularly useful for determining the location, depth and/or diameter of reinforcement bars or other metal parts within concrete.